It's possible to bypass standard dpkg confile handling and replace it with other tools called in postinst.

cme and Config::Model

See https://ddumont.wordpress.com/2014/03/23/easier-lcdproc-package-upgrade-with-automatic-configuration-merge/ and https://wiki.debian.org/PackageConfigUpgrade.

ucf

The good thing about ucf is that it has support for 3 ways merge which would be nice for at least xwiki.cfg and xwiki.properties.

See documentation at http://man.he.net/man1/ucf. Also see http://anonscm.debian.org/cgit/collab-maint/foomatic-filters.git/tree/debian/foomatic-filters.postinst for an example of how to use it.